A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In the semiconductor manufacturing industry, there is an increasing demand for ever-smaller features and increased density of features. Critical dimensions (CDs) of pattern features are therefore rapidly decreasing, and are becoming very close to the theoretical resolution limit of state-of-the-art lithographic apparatus such as the steppers and scanners as described above. Conventional techniques for enhancing resolution and minimizing patternable critical dimension include: reducing the wavelength of the exposure radiation; increasing the numerical aperture of the projection system of the lithographic apparatus; and including features smaller than the resolution limit of the lithographic apparatus so that they will not be patterned onto the substrate, but so that they will produce diffraction effects which can improve contrast and sharpen fine features of patterns applied to the substrate. However, application of such conventional resolution enhancement techniques may lead to a reduction of depth of focus within which, for example, imaging of desired patterns at or near the limit of the resolution capability can be achieved. A reduced depth of focus may lead to pattern defects (for example, the blurring of lines or edges of pattern features) beyond tolerance when, for example, a residual substrate unflatness cannot be compensated for during exposure of the substrate.
It is desirable to provide, for example, a method and apparatus that obviates or mitigates one or more of the problems identified above, or one or more of the problems of the prior art in general, whether identified herein or elsewhere. For instance, it is desirable to provide a method and apparatus which increases the depth of focus of a pattern feature imaged by a lithographic apparatus.